The invention relates to a process for converting hydrocarbons. The process of the invention associates a distillation zone with a hydrocarbon conversion reaction zone which is at least partially external to the distillation zone into which an effluent comprising hydrogen is introduced. Thus this process can selectively convert hydrocarbons, separated from a hydrocarbon feed by means of the distillation zone, in an associated reaction zone, withdrawing the feed for the reaction zone from the distillation zone and re-introducing the converted feed into the distillation zone.
More particularly, the process of the invention is applicable to selective reduction of the quantity of light unsaturated compounds (i.e., containing at most six carbon atoms per molecule) comprising optional olefins and benzene, of a hydrocarbon cut essentially comprising at least 5 carbon atoms per molecule, with no substantial loss of octane number.
The general trend now is to reduce the quantity of benzene and olefins (unsaturated compounds) in gasolines, because of their known toxicity.
Benzene has carcinogenic properties and thus the possibility of it polluting the air must be limited as far as possible, in particular by practically excluding it from automobile fuels. In the United States, reformulated fuels must not contain more than 1% by volume of benzene; in Europe, it has been recommended that a gradual decrease towards that value be made.
Olefins are acknowledged as being among the most reactive hydrocarbons in the photochemical cycle of reactions with oxides of nitrogen, which occur in the atmosphere and which leads to ozone formation. A rise in the ozone concentration in the air may be a cause of respiratory problems. A reduction in the olefin content in a gasoline, more particularly of the lightest olefins which have a greater tendency to evaporate when manipulating the fuel, is thus desirable.
The benzene content of a gasoline is very largely dependent on that of the reformate component in that gasoline. The reformate results from catalytic treatment of naphtha intended to produce aromatic hydrocarbons, principally comprising 6 to 9 carbon atoms per molecule and the octane number of which is very high endowing the gasoline with antiknock properties.
Because of the toxicity described above, the amount of benzene in the reformate must be reduced by a maximum.
The benzene in a reformate can be hydrogenated to cyclohexane. Since it is impossible to selectively hydrogenate benzene in a mixture of hydrocarbons also containing toluene and xylenes, that mixture must first be fractionated to isolate a cut containing only benzene, which can then be hydrogenated.
International patent application WO 95/15934 describes a reactive distillation which aims to selectively hydrogenate diolefins and C2-C5 acetylenic compounds. The catalytic hydrogenation zone is completely internal to the distillation column, which means that the hydrogen cannot dissolve properly in the feed and the pressure cannot be increased.
A number of processes have been described in which the catalytic benzene hydrogenation zone is internal to the distillation column which separates benzene from other aromatic compounds, which cuts the cost of the apparatus. Such processes are described in United States patents U.S. Pat. Nos. 4,232,177, 4,307,254, 4,336,407, 3,629,478, 4,471,154 and 3,629,478. It appears that the pressure drop across the catalytic bed(s) in that process means that an intimate mixture between the liquid phase and the gaseous stream containing the hydrogen cannot be obtained. In that type of technology where the reaction and distillation proceed simultaneously in the same physical space, the liquid phase descends through every catalytic bed in the reaction zone in a trickle flow, and thus in threads of liquid. The gaseous fraction containing the fraction of vaporised feed and the gas stream containing hydrogen rise through the catalytic bed in columns of gas. In that arrangement, the entropy of the system is high and the pressure drop across the catalytic bed(s) is low. As a result, operating that type of technique cannot easily promote dissolution of hydrogen in the liquid phase comprising the unsaturated compound(s).
A number of processes have been described in which the reaction zone is external to the distillation column with the feed to be converted being withdrawn at one level of the column and the converted effluent being re-introduced into the column. Such processes are described in U.S. Pat. No. 4,503,265 and in International applications WO 93/19031, WO 93/19032 and WO 94/13599 for application to the synthesis of alkylethers. Similarly, U.S. Pat. No. 5,177,283 describes this technique for alkylating aromatic hydrocarbons.
The Applicant""s European patent application EP-A1-0 781 830 describes a process for hydrogenating benzene using a distillation column associated with a reaction zone which is at least partially external. The feed for the reaction zone is withdrawn from the distillation zone then the effluent from the reaction zone is re-introduced into the distillation zone. A distillate is recovered from the head of the distillation zone and a reformate is recovered from the bottom of the distillation zone.
The process of the present invention is an improvement over the Applicant""s patent application EP-A1-0 781 830, the features of which are hereby included in the present description.
The invention provides a process for converting a hydrocarbon feed associating a distillation zone producing an overhead distillate and a bottom effluent and a reaction zone which is at least partially external to the distillation zone. At least one reaction for converting at least a portion of at least one hydrocarbon takes place in a reaction zone comprising at least one catalytic bed, in the presence of a catalyst and of a gas stream comprising hydrogen. The feed for the reaction zone is drawn off from the distillation zone at the height of a draw-off level and represents at least a portion of the liquid flowing in the distillation zone, and at least a portion of the effluent from the reaction zone is re-introduced into the distillation zone at the height of at least one re-introduction level, so as to ensure continuity of distillation. The invention is characterized in that at least one liquid effluent is also withdrawn from the distillation zone at the height of at least one withdrawal level, at least a portion of said liquid effluent being at least partially treated in a gas-liquid side separation zone (splitter) wherein at least part of the gaseous effluent is re-introduced into the distillation zone and wherein at least part of the liquid effluent is recovered as an intermediate cut.
Thus the process of the present invention can recover at least one hydrocarbon cut at an intermediate level in the distillation zone, i.e., at a level between the bottom and the head of the distillation zone, with quantities of the desired products which can be adjusted as required, said cut being free of the major portion of the lightest compounds at least part of which are re-introduced into the distillation zone after separation in the gas-liquid side separation zone. The association of a distillation zone and a reaction zone means that the process of the invention enables products to be to separated using the distillation zone and to specifically convert certain compounds, under advantageous temperature and pressure conditions, in order to recover a distillate from the head of the distillation zone in which the major portion of the hydrocarbons to be converted has been converted and to recover at least a fraction of the intermediate hydrocarbons with the desired hydrocarbon composition from the conversion reaction, of hydrocarbons present in the feed from the distillation zone and of other compounds introduced for the conversion reaction, from any point of the distillation zone.